Asphalt compositions, such as asphalt concrete used for paving roadways or asphalt roofing compositions, comprise an asphalt cement and aggregate. The aggregate is typically natural or synthetic sand and gravel of various sieve sizes or specified meshes. The asphalt may be of a variety of grades, for example, harder grades of asphalt will be used for pavement applications in warmer climates while softer grades of asphalt will be used for pavement applications in colder climates. The asphalt may be modified by inclusion of polymeric materials, such as rubber or latex. Likewise, the mesh distribution and type of aggregate may vary widely, depending upon the application. In asphalt concrete used for paving, the aggregate is by far the major portion, typically comprising about 95 weight percent with the asphalt portion typically comprising the remaining 5 weight percent.
As described in U.S. Pat. Nos. 4,775,419, 4,430,127, 4,639,273, 4,743,304 and 5,037,474, the teachings of each of which are incorporated herein by reference, a major limitation to durability of asphalt compositions is the strength of the bond between the asphalt portion and the aggregate, a bond which is weakened by water, either water initially present in the aggregate or water to which the asphalt composition is subsequently exposed. Accordingly, as taught by the above-referenced patents, an anti-stripping agent is incorporated into the asphalt portion. The anti-stripping agent is typically an organic polyamine, e.g., U.S. Pat. No. 4,430,127, but may be a variety of other compounds or mixtures of compounds. U.S. Pat. No. 4,775,419 teaches the use of metal amine complexes. U.S. Pat. No. 4,430,127 teaches epoxylated polyamines. U.S. Pat. 4,639,273 teaches formaldehyde adducts of amines, polyamines and amides. U.S. Pat. No. 4,743,304 teaches the use of imadazolines, polyamines, alkoxylated polyamines, aminocarboxylic esters, amide-amines and mixtures thereof. U.S. Pat. No. 5,037,474 teaches using the reaction product of an epoxy compound with either phosphoric acid or a partial ester of phosphoric acid. Other materials, such as lime added to the aggregate, or fillers, such as cellulose or portland cement, may also be added to increase the strength and durability of the asphalt composition.
The lifetime of an asphalt composition is heavily dependent upon the additives added, particularly anti-stripping agent, but also lime and fillers. Accordingly, contracts, such as state paving contracts, will typically specify the amount and type of additives to be used. Unfortunately, there is no particularly good way to determine the content of anti-stripping agent and other additives once the components of the asphalt compositions are blended together. Additives, such as anti-stripping agent comprise only a very minor proportion of the asphalt concrete. Anti-stripping agent typically comprises between about 0.2 and about 5.0 wt. percent of the asphalt portion, which, in turn comprises about 54 of the asphalt concrete. Although additives are minor components, on a cost per pound basis they are generally considerably more costly than either the aggregate or asphalt itself. Honest mistakes are made in mixing components of an asphalt composition. Unfortunately, there is an economic incentive for a potentially dishonest contractor to reduce or eliminate anti-stripping agent or other additives from the paving composition or to use a cheaper grade of anti-stripping agent than specified by the contract. The temptation to cheat in this regard is increased by the lack of efficient test methods for determining the amount of additives. Even without a pre-intent to defraud, contractors have been known to continue paving operations after running out of additives so as to avoid work stoppages.
It would be desirable for the specifying party, such as the state or ministry, contracting for asphalt paving, to have a way of monitoring the level of additives actually used to ensure compliance with the terms of the contract. This might be done by sampling the mix while the asphalt portion is still hot or sampling the hardened asphalt concrete (hot mix asphalt), either at the time the roadway is paved, or at a subsequent time should testing be warranted or necessary.
Most efforts in measuring anti-stripping agents have centered on chemical analysis. Efforts to date have produced tests which are cumbersome to perform, poorly quantitative, and generally incapable of distinguishing between anti-stripping agents of various qualities. The present invention is directed to tagging asphalt additives and identifying the tagged additive in the asphalt.